ip accounting-list

Cisco has interface level command "ip accounting" which records number of bytes and packets passed through the router. If you want to count traffic only for specific IP address, you need to use "ip accounting-list" command. There seems to be a tiny bug in context help in 12.4(15)T14:

R2(config)#ip accounting-list 1.1.1.1 ?
  A.B.C.D  IP address mask

Note that context help says you need to enter address mask. 

R2(config)#ip accounting-list 1.1.1.1 255.255.255.255

After checking

R2(config)#do sho run | i accounting

ip accounting-list 0.0.0.0 255.255.255.255

That's not what I entered. In reality, you are supposed to enter wildcard mask, which makes more sense. Specifying ACL number or name would have made even more sense. So, let's fix it:

R2(config)#no ip accounting-list 0.0.0.0 255.255.255.255

R2(config)#ip accounting-list 1.1.1.1 0.0.0.0

R2(config)#interface fa 0/0

R2(config-if)#ip accounting

and test it. Traffic from R3 to 192.168.12.1 and 1.1.1.1 must pass through R2's Fa0/0 interface: 

R3#ping 192.168.12.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.12.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 28/58/120 ms

R3#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 16/47/88 ms

R2#sho ip account

   Source           Destination              Packets               Bytes

 192.168.23.3     1.1.1.1                          5                 500

As you can see from the output above, only pings to 1.1.1.1 got counted.